Cellular telecommunications networks include a network of base stations which communicate with User Equipment (UE) within a particular coverage area. Traditional base stations are often known as macrocells (owing to their relatively large coverage area over several kilometers squared), but modern cellular networks also include small cells (including femtocells, picocells, microcells and metrocells) which have smaller coverage areas than the macrocell. The use of these different technologies allows Network Operators to increase capacity by deploying the small cell base stations to a large number of customer premises (e.g. homes and businesses), thereby increasing capacity where there is demand.
Despite the advantages of a mass deployment of small cell base stations within a heterogeneous network, Network Operators are conscious of the increased energy demand of such a large number of base stations. Accordingly, the 3rd Generation Partnership Project (3GPP) introduced an energy saving feature in Technical Specification (TS) 36.300, section 22.4.4. In this standard, base stations were designated as either “coverage” or “capacity”. The coverage base station provides basic service about a large coverage area but also controlled one or more capacity base stations. The coverage base station may therefore switch the capacity base stations between normal and energy saving modes of operation according to an energy saving policy (such that the capacity base stations enter energy saving mode when demand is low). The capacity base stations were also allowed to autonomously switch between the normal and energy saving mode, but under a policy set by the coverage base station. The switch off instruction or the policy may also come from the Network Operator via the coverage base station. Nonetheless, each base station had one of two defined roles in which it could operate.
The energy saving function is further defined in 3GPP TS 32.551. This specification discusses the problem of coverage holes being created when capacity base stations enter energy saving mode. Accordingly, base stations are also able to enter “compensation” mode, in which the base station serves one or more UEs previously served by the base station that has entered energy saving mode. This may be achieved by a simple handover of the UEs to the coverage base station, or may be a handover to a new base station (e.g. a neighboring base station). The compensation base station may also require a substantial change in its coverage area in order to serve the new UEs.
A further 3GPP specification, TR 36.927, discusses potential solutions for energy saving and highlights policies for “re-activation” of base stations (i.e. exiting energy saving mode). These policies include:    No assistance—in which the coverage base station instructs the capacity base station to exit energy saving mode without any knowledge of its local conditions;    Reactivation based on load;    Reactivation based on Interference over Thermal (IoT) measurements;    Reactivation based on UE measurements; and    Reactivation based on UE and base station locations.